1. Field of the Invention
The present invention relates to a method and apparatus for grinding cams having a re-entrant surface with a grinding wheel.
2. Discussion of the Related Art
Heretofore, there has been known a method of grinding cams on a workpiece while the rotation about a C-axis of the workpiece carried on a workpiece support device and the feed in an X-axis direction of a wheel head carrying a grinding wheel are controlled by a numerical controller in a simultaneous two-axis control mode. In recent years, requirements in cam shape have become complicated and highly precise with the increase in performance of engines, and there is an increasing demand for cams with a re-entrant surface (A) as shown in FIG. 4. A small diameter grinding wheel which is able to contact with the re-entrant surface (A) is necessary in grinding cams having the aforementioned re-entrant surface. However, an interference takes place between the wheel head and the workpiece support device where the diameter of the grinding wheel is made to be small in the structure that the rotational axes for the workpiece and the grinding wheel are kept in parallel. To obviate this, in a cam grinding machine described in Japanese unexamined, published patent application No. 8-243906, a workpiece support device for supporting a workpiece and a wheel head carrying a grinding wheel are mounted to be movable respectively in Z and X-axis directions perpendicular to each other, a wheel spindle is mounted on the wheel head with its axis extending inclined with respect to the X-axis within an X–Z axis plane, and a small-diameter, tapered grinding wheel is mounted on one end of the wheel spindle to extend its generating line in parallel to the Z-axis.
Where a cam on a workpiece is ground by moving the grinding wheel back and forth in the X-axis in dependence on the rotational angle of the workpiece about the C-axis, in order that the grinding wheel with a predetermined diameter is able to contact with a desired cam shape at each rotational phase of the workpiece, the grinding wheel is given a plunge grinding feed toward the workpiece in the X-axis while the rotation about the C-axis of the workpiece and the movement in the X-axis of the wheel head are cooperated under the simultaneous two-axis control in accordance with cam profile generating NC data. In the cam profile generation motion, the point where the grinding wheel contacts with the desired cam shape moves up and down across a line segment connecting the rotational center of the workpiece with the rotational center of the grinding wheel, in dependence on the inclination angle of a common tangential line at the point. For this reason, where the grinding wheel has its diameter which differs from that set in preparing the cam profile generating NC data, an error occurs on the cam profile of the cam which is ground with the grinding wheel. For example, in the cam grinding apparatus in the aforementioned Japanese unexamined, published patent application No. 8-243906, where the cam profile generating NC data is prepared by taking as a predetermined diameter a nominal wheel diameter within a plane which is normal to the Z-axis at a center portion of the tapered grinding wheel and then, a plunge grinding is performed while the profile generating motion is given in accordance with the NC data, profile errors as shown in FIG. 5 are made on the ground cam shape due to the fact that the nominal diameter and the predetermined diameter differ at each of the small-diameter and large-diameter sides of the tapered grinding wheel.
In order to preclude the aforementioned profile errors, an attempt may be made in the cam grinding machine described in the aforementioned Japanese unexamined, published patent application No. 8-243906 to mount on an end of the wheel spindle a small-diameter spherical grinding wheel having its center on the axis of the wheel spindle and to make the wheel head perform a traverse grinding feed relative to the workpiece support device in the Z-axis direction while the workpiece and the wheel head perform the cam profile generating motion. However, where the attempt is made, the infeeding at the traverse feed end of the grinding wheel against the workpiece in the X-axis direction and the traverse grinding feeding have to be repeated many times, thereby resulting in a long grinding time.